The subject matter disclosed herein relates to helicopter blades. More specifically, the subject disclosure relates to construction and methodology of construction of a composite spar for helicopter blades.
Typical helicopter composite blade spars have spanwise and chordwise stiffness and structural requirements. The upper and lower airfoils of the blade are generally the most effective in generating the high bending and axial stiffnesses required for spanwise bending and centrifugal loading conditions. This results in the incorporation of unidirectional graphite and fiberglass plies in the blade spars in the spanwise direction. The total thickness and combination are a function of strength and dynamic requirements. Additionally, the torsional loading in the blade is most efficiently reacted by +/−45 degree graphite which result in a torque reaction in the chordwise direction around the airfoil. Therefore a typical helicopter blade will have a +/−45 degree graphite ply laminate at inner and outer mold lines of the blade.
Typical helicopter composite blade spars are fabricated over a foam mandrel encased in a rubber bladder by a layup process of multiple plies of composite material. The assembly is put in a metal mold and external temperature and pressure from the rubber bladder cures the composite laminate. During the ply layup process, individual plies are not overlapped at a trailing edge backwall region for ease of fabrication. This process, as shown in FIG. 3, leads to overlapping an entire upper airfoil laminate 100 with an entire lower airfoil laminate 102 creating a scarf joint 104 at a backwall 106 of the composite spar. This leads to a spar backwall resin plane extending from the upper airfoil laminate 100 to lower airfoil laminates 102. The torsional loading in the chordwise direction of the spar subjects the weak resin plane to critical interlaminar stresses not using the high strength of the composite fibers. A crack initiation or manufacturing defect could cause a sudden disbond of the entire backwall scarf joint 104 leading to serious consequences for the safety and structural integrity of the composite blade.